re-enable computed goto; fix ,help in the repl; subr dispatch optimizations

* m4/labels-as-values.m4: New file, checks for computed goto.

* configure.in: Use AC_C_LABELS_AS_VALUES.

* module/system/repl/command.scm (procedure-documentation): Extend the
  core's procedure-documentation in an ad-hoc way, so that ,help works.

* module/system/vm/core.scm (program-properties): New function.
  (program-documentation): New function.

* src/vm_engine.h (DROP, DROPN): Decrement sp before checking for
  underflow.

* src/vm_system.c (call, tail-call): Add some optimized dispatch for some
  C functions, so that we can avoid consing and the interpreter if
  possible. However currently it seems that I'm always getting the
  scm_call_* trampolines back.

module/system/repl/command.scm

@@ -55,6 +55,14 @@
 (define (group-name g) (car g))
 (define (group-commands g) (cdr g))
 
+;; Hack, until core can be extended.
+(define procedure-documentation
+  (let ((old-definition procedure-documentation))
+    (lambda (p)
+      (if (program? p)
+          (program-documentation p)
+          (procedure-documentation p)))))
+
 (define *command-module* (current-module))
 (define (command-name c) (car c))
 (define (command-abbrev c) (if (null? (cdr c)) #f (cadr c)))

module/system/vm/core.scm

@@ -24,6 +24,7 @@
   :export (arity:nargs arity:nrest arity:nlocs arity:nexts
            make-binding binding:name binding:extp binding:index
            program-bindings program-sources
+           program-properties program-property program-documentation
            frame-arguments frame-local-variables frame-external-variables
            frame-environment
            frame-variable-exists? frame-variable-ref frame-variable-set!
@@ -66,10 +67,16 @@
   (cond ((program-meta prog) => cadr)
 	(else '())))
 
+(define (program-properties prog)
+  (or (and=> (program-meta prog) cddr)
+      '()))
+
 (define (program-property prog prop)
-  (cond ((program-meta prog) => (lambda (x)
-                                  (assq-ref (cddr x) prop)))
-	(else '())))
+  (assq-ref (program-properties proc) prop))
+
+(define (program-documentation prog)
+  (assq-ref (program-properties proc) 'documentation))
+
 
 
 ;;;